Allen-Bradley 1770-LDA Product Data page 11

6. Check that power supply DC voltages are present on module
receptacle pins.
7. Check that line driver/receiver front panel LED indicator is on.
Indicator goes out if the remote I/O interconnect cable opens or if the
far end line driver/receiver loses its operating voltages.
8. Check for an open remote I/O interconnect cable or shorted cable.
This is not indicated on the industrial terminal screen when using a
1772-LN1, -LN2, -LN3, -LP1, -LP2 PC processor with a 1770-T1,
-T2 industrial terminal.
9. Check for an open remote I/O interconnect cable or shorted cable. it
will cause the following message to be displayed on the screen:
Processor Communications Link Noisy Alphanumeric Enabled Only,
when using a 1772-LN1, -LN2, -LN3, -LP1, or -LP2 processor or a
1772-LV or 1772-LP3 processor and the 1770-T3 industrial terminal.
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